Studies have shown that the antiviral activity of ZDV can continue in the body even after there does not appear to be any ZDV left in the blood. This occurs because the body breaks down the drug into substances that remain active against HIV. The body also breaks down 3TC, a drug that is combined with ZDV in the Combivir product, in a similar way. Since antiviral activity may continue after Combivir is removed from the body, it may not be necessary to take the drug as often as once thought. This study carefully measures levels of the active substances in order to find out whether the same amount of antiviral activity occurs with less-frequent dosing.